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Twin laws of whewellite, CaC2O4·H2O. A structural and growth approach

Abstract

Both natural and synthetic crystals of whewellite (CaC2O4·H2O, sp.g. P21/c) occur commonly as twins. The geometrical, reticular and structural features of the most important twin law, twin plane (100), and the dubious law (\(\overline {\text{1}} \)01) were investigated. The strict application of the periodic bond chain (PBC) theory (Hartman and Perdok 1955) makes possible the identification of original composition planes (OCP). Following the OCP analysis it has been deduced that the energy necessary to produce a (100) twin is very low. This is evidenced by the easy reciprocal arrangement of the two crystal structures which can be shown in two slightly different ways, i.e., by a (100) reflection with a mean [0, 1/2, −a 0cosβ] translation and a weak relaxation (from 0.1 to 0.25 Å) of the common interface. In OCP an important role is played by water molecules. With regard to (\(\overline {\text{1}} \)01) twinning, the geometrical and reticular agreement is not matched by the structural situation. The structural and growth approach to the twin laws of whewellite therefore justifies the very common occurrence only of (100) twinned crystals.

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Aquilano, D., Franchini-Angela, M. Twin laws of whewellite, CaC2O4·H2O. A structural and growth approach. Phys Chem Minerals 7, 124–129 (1981). https://doi.org/10.1007/BF00308228

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  • DOI: https://doi.org/10.1007/BF00308228

Keywords

  • Reflection
  • Crystal Structure
  • Water Molecule
  • Structural Feature
  • Mineral Resource